That's not actually true at all. US aircraft suffered massively during the Vietnam war, the perils of heliborne and close in CAS operations.
en.wikipedia.org
Over 11,000 aircraft is not an insignificant amount to loss (nearly half of all helicopters in theatre).
They hardly ran out of "all the good shit" almost instantly, they shot down US planes like fly's for basically 6 years and these are America's numbers on losses....
Also the North Vietnamese air force had an incredible amount of air aces, those mig21's got really fed up of shooting down F4's and F105's.
https://en.wikipedia.org/wiki/List_of_aircraft_losses_of_the_Vietnam_War
As much as people like to ape on the Russian air force they certainly haven't lost that many planes in Ukraine even if the war theoretically went on for 6 years, and remember the only plane to shoot down an F18 was an old Mig 25 Foxbat (lovely looking plane).
Iraqi MiG-25 Foxbat Pilot explains how he was able to shoot down Lt Cdr Speicher’s F/A-18 Hornet the first night of Operation Desert Storm
theaviationgeekclub.com
Sorry for using wiki as the reference it was just the fastest source but many other places cover the info.
Do some research on past wars before spouting misinformation for those not in the know.